Audio channel fault detection system

ABSTRACT

An audio system includes a diagnostic capability to check for faults to a power source and faults to ground for output audio channels configured to drive loudspeakers. The fault to ground analysis involves analysis of a number of digital samples to determine if a predetermined threshold is exceeded during a predetermined period of time. The analysis may involve both a digital signal processor and a microprocessor performing a zero crossing analysis using the predetermined threshold and the predetermined window of time.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional application Ser.No. 61/642,708 filed May 4, 2012, the disclosure of which is herebyincorporated in its entirety by reference herein.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to audio systems, and more particularly toan audio channel fault detection system for use with an audio system.

2. Related Art

Audio systems typically include a source of audio content such as anaudio file, compact disc player, or digital video disc (DVD) player thatprovides an audio signal, an amplifier to amplify the audio signal, andone or more loudspeakers driven by the amplifier to produce the audiosignal as audible sound. The amplifier and loudspeakers are typicallyinterconnected with wiring to transmit the amplified audio signals. Theamplified audio signals can be high electrical current and/or voltagesignals. When the circuit between the amplifier and loudspeakers iscompromised, such as by damage to the wiring, a short to ground, or ashort to a power supply of the audio system a fault may occur. Underfault conditions, faulty operation and/or damage to the audio system mayresult.

SUMMARY

An audio system may perform audio channel fault detection during amonitoring period, such as during startup, or during operation, of apower amplifier included in the audio system. The fault detection mayinvolve monitoring a feedback current signal representative of an outputelectrical current present on respective output audio channels of thepower amplifier. Detection of a ground fault may be based on thefeedback current signal being outside a predetermined threshold for apredetermined period of time during the monitoring period.

The fault detection may be performed by a signal processor that includesa microprocessor performing logic based functionality in cooperativeoperation with a digital signal processor performing sampling basedfunctionality. The sampling based functionality may involvesample-by-sample analysis of the feedback current signal to identifyzero crossings of an alternating electrical current wave formrepresentative of the feedback current signal. Sampling functionalitymay also involve tracking and storing a number of samples representingan electrical current on the audio output channels during a monitoringperiod, such as during the startup of the power amplifier or operationof the power amplifier. The number of zero crossings during themonitoring period may be used to determine if a fault to ground exists.

The predetermined period of time may be long enough to avoid falsedetection of a ground fault during other operational events occurring inthe audio system. The predetermined threshold may be a parameter used inconjunction with the predetermined period of time to identify anelectrical current event on one or more output audio channels. Inaddition, during the predetermined period of time, while the feedbackcurrent is outside the predetermined threshold, a length of time betweenzero crossings, or a number of zero crossings among the samples may bedetermined and stored. A number of samples representing the initiationof the electrical current event to the conclusion of the electricalcurrent event may be stored to identify the predetermined window oftime, and the length of time between zero crossings or the number ofzero crossings indicated within the number of samples may be confirmedas being below a predetermined threshold to indicate that the electricalcurrent event is a short to ground event.

Alternatively, or in addition, the predetermined threshold may be usedto trigger a count of samples of the alternating electrical current onthe audio channels that is outside the predetermined threshold using alocal counter. The local counter may count samples of the electricalcurrent until a zero crossing of the alternating electrical current isdetected, at this time the local counter may be reset. Prior to reset, acount value in the local counter may be compared to a maximum countregister value. If the local counter value exceeds the maximum countregister value, the maximum count register value may be updated with thecount value from the local counter. The maximum count register value maybe compared to a threshold value representing the predetermined periodof time. If the maximum count register exceeds the threshold value, ashort to ground event is indicated.

Other systems, methods, features and advantages of the invention willbe, or will become, apparent to one with skill in the art uponexamination of the following figures and detailed description. It isintended that all such additional systems, methods, features andadvantages be included within this description, be within the scope ofthe invention, and be protected by the following claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be better understood with reference to the followingdrawings and description. The components in the figures are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention. Moreover, in the figures, likereferenced numerals designate corresponding parts throughout thedifferent views.

FIG. 1 is a block diagram of an example audio system.

FIG. 2 is a block diagram of an example signal processor that may beincluded in the audio system of FIG. 1 to perform short to power sourcediagnostics.

FIG. 3 is a block diagram of example operation of the signal processorincluded in FIG. 2 during an example short to power source event.

FIG. 4 is a plot of an example short to ground within an example audiosystem.

FIG. 5 is a plot of an example electrical current burst event within anexample audio system.

FIG. 6 is a block diagram of another example signal processor that maybe included in the audio system of FIG. 1 to perform short to grounddiagnostics.

FIG. 7 is an example operational block diagram of the signal processorof FIG. 6.

FIG. 8 is an operational flow diagram describing example operation of adigital signal processor included in the signal processor of FIG. 6.

FIG. 9 is an operational flow diagram describing example operation of amicroprocessor included in the signal processor of FIG. 6.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a block diagram of an example audio system 100 that includesan audio source 102, a signal processor 104, and at least oneloudspeaker 106. The audio system 100 may be any system capable ofproviding audio content such as a multimedia system in a vehicle.Examples of the audio source 102 include a compact disc player, a videodisc player, a radio tuner, a navigation system, a mobile phone, avehicle head unit, a wireless or wireline communication device, apersonal computer, a multimedia memory storage device, such as an IPODor MP3 player, or any other device capable of generating digital oranalog audio signals representative of audio sound. In one example, theaudio source 102 may provide digital audio input signals representativeof left and right stereo audio input signals on left and right audioinput channels. In other examples, the audio input signal may be analogsignals. Alternatively, or in addition, the audio input signals may bereceived as microphone input signals, received as streaming audiosignals over a network, such as the Internet, or be generated as livesound audio signals. The audio signal may include any number ofchannels, such as a mono audio input channel, seven audio input channelsin Logic 7™ surround sound, or six audio channels in Dolby 5.1™ surroundsound.

The signal processor 104 may be any computing device capable ofprocessing audio and/or video signals, such as a computer processor, adigital signal processor, a microprocessor and the like. The signalprocessor 104 may operate in association with a memory 110 to executeinstructions stored in the memory 110. The instructions may provide atleast part of the functionality of the audio system 100. The memory 110may be any form of one or more data storage devices, such as volatilememory, non-volatile memory, electronic memory, magnetic memory, opticalmemory, or any other form of non-transitory data storage mechanism orsystem. The memory 110 may store instructions and data. The data may beparameters used/updated during processing, parameters generated/updatedduring processing, user entered variables, and/or any other informationrelated to processing audio signals.

The signal processor 104 may be one or more processing devices capableof performing logic to process the audio signals supplied on one or moreaudio channels from the audio source 102. Such processing devices mayinclude digital signal processors (DSP), microprocessors, fieldprogrammable gate arrays (FPGA), or any other device(s) capable ofexecuting instructions. In addition, the audio signal processor 104 mayinclude other signal processing components such as filters,analog-to-digital converters (A/D), digital-to-analog (D/A) converters,signal amplifiers, decoders, delay, or any other audio processingmechanisms. The signal processing components may be hardware based,software based, or some combination thereof.

In FIG. 1, the signal processor 104 includes a processing device 114 inthe form of a digital signal processor (DSP) 116 cooperatively operatingwith a microprocessor 118 to execute the instructions stored in thememory 110. In one example, the DSP 116 may be used for sampling theaudio signal and performing audio signal related processing in thedigital domain. The microprocessor 118 may perform logic basedcomputations related to processing the audio signals. Audio signalsreceived by the signal processor 104 on one or more audio channels 120may be converted to the digital domain (if not received in digital form)using an analog-to-digital converter (ADC) 122 and provided to themicroprocessor 118 and/or the DSP 116 for processing. Theanalog-to-digital converter 122 may be a separate device, or may beinstructions executed by the processing device 114, and may includefiltering to minimize or eliminate any direct current (DC) componentincluded in the audio signals, since the audio signals are alternatingelectrical current signals. Processing by the processing device 114 mayinclude equalization, audio signal modification, such as delay, phaseadjustment, frequency based signal processing, routing or any other formof audio signal processing of the audio signals. Signal processing, suchas filtering, noise compensation, loudness, buffering, or any other formof signal modification may also be performed in the signal processor 104external to, internal to, or in conjunction with the processing device114.

The signal processor 104 may also include a power amplifier 126. Thepower amplifier 126 may receive and amplify processed audio signals toincrease the amplitude of the audio signals received. The processedaudio signals may be supplied to the power amplifier 126 on a processedaudio signal line 128. The processed audio signals may be digital audiosignals. The digital audio signals may be converted to analog audiosignals by a digital-to-analog converter (DAC) 130 and provided asdigital processed audio signals on a digital processed audio signal line132. Digital processed audio signals may be received and amplified by anamplifier module 134, and output as amplified audio output signals onoutput audio channels 136 to the one or more loudspeakers 106. Thedigital-to-analog converter 130 may be a separate device, or may beinstructions executed by the processing device 114. Alternatively,analog processed audio signals may be supplied from the processingdevice 114, and the DAC 130 may be omitted. The amplifier module 134 maybe any form of signal amplification device, such as a class D audioamplifier. In one example, the amplifier module 134 may be a powerintegrated circuit. The term “module” may be defined to include one ormore executable modules. The modules are defined to include software,hardware or some combination thereof executable by the signal processor104. Software modules may include instructions stored in the memory 110,or other memory device, that are executable by the signal processor 114or other processor. Hardware modules may include various devices,components, circuits, gates, circuit boards, and the like that areexecutable, directed, and/or controlled for performance by the signalprocessor 114.

The one or more loudspeakers 106 may be any form of transducer devicecapable of translating electrical audio signals to audible sound. Theloudspeaker 106 may be a group of loudspeakers 106 that are configuredand located to operate individually or in groups, and may be in anyfrequency range. The one or more loudspeakers 106 may collectively orindividually be driven by amplified output channels 136, or amplifiedaudio channels, provided by the signal processor 104. The loudspeakers106 may consist of a heterogeneous collection of audio transducers thatreceives a number of separate audio channels, such as stereo, 5 channel,6 channel or seven channel audio signals. Each transducer may receive anindependent and possibly unique amplified output audio signal from thesignal processor 104. Accordingly, the audio system 100 may operate toproduce mono, stereo, or surround sound signals using any number ofloudspeakers 106.

In addition to processing audio signals, the signal processor 104 mayalso provide fault diagnostic testing of the output audio channels 136.The fault diagnostic testing may include 1) short-to-power source (STS)diagnostics; and 2) short to ground (STG) diagnostics. In the example ofa vehicle, the power source is typically a DC (direct current) powersource that includes a battery, however any power source may be present.Thus, in a short to power source (STS) scenario, the power sourcepotential is typically the positive side of the power source. Inaddition, in the example of a vehicle, ground potential in a short toground (STG) scenario is typically at the potential of the negative sideof the power source. Faults in the form of STS and STG may occuranywhere from the output channels of the power amplifier 126 to includewithin the loudspeaker(s) 106 when an undesired electrically conductivepath is formed to either the power source in an STS scenario, or toground potential in an STG scenario.

Fault diagnostics to check for STS and STG may be performed using thesignal processor 104 at the time the audio system 100 is initiallyenergized using instructions from memory 110 executed by the processingdevice 114. Alternatively, or in addition, fault diagnostics may beperformed during operation of the audio system 100 following startup.Accordingly, no dedicated or specialized integrated circuits or hardwaredevices are necessary for detecting fault conditions (STS or STG), sincethe fault diagnostics are performed by the processing device 114 usingsoftware stored in the memory 110. Although the remaining discussionwill focus on a specific application, namely, an audio system in avehicle, the features and functionality used herein may be applied inany other system that includes an audio source, an amplifier andloudspeakers.

FIG. 2 is a block diagram example of the signal processor 104 thatincludes a processing device 114 and a power amplifier 126. The poweramplifier 126 may include an audio output channel DAC 202 that receivesfrom the processing device 114 processed digital audio signals on aprocessed digital audio signal line 204, and provides processed analogaudio signals on a processed audio analog audio signal line 206 to theamplifier module 134.

The amplifier module 134 may receive supply power (VP), such as +2.5VDCand −2.5VDC from a power supply 210 on power supply lines 212. In oneexample, the power supply 201 may be a tracking power supply thatautomatically adjusts an output voltage and electrical current of thepower supply in response to the amplitude of the audio signals processedby the processing device 114. In addition, an indication of the outputof the power supply 210 may be provided as a feedback signal to theprocessing device 114 on a feedback signal line 214 to an ADC 216, whichmay be internal or external to the processing device 114. In otherexamples, the feedback signal may be analog or digital, and noconversion may be needed. The feedback signal may be provided to a powersupply feedback module 218.

The processing device 114 may also provide a digital power supplycontrol signal from a power supply control module 220 on a power supplycontrol line 222. The power supply control module 220 may be included inthe processing device 114, and may control operation of the power supply210. The digital power supply control signal may be converted to analogby a DAC 224, and provided as an analog power supply control signal onan analog power supply control signal line 226. In other examples, thesesignals may be analog or digital, and no conversion may be needed. Thepower supply control signal may control the voltage output of the powersupply 210 based at least in part on the feedback signal. Alternatively,or in addition, where the power supply 210 is a tracking power supplywith adjustable voltage and electrical current output, the power supplycontrol signal may provide a control signal in accordance with theamplitude of the audio signals being processed and the feedback signal.The tracking power supply 210 of this example may adjust the outputvoltage and electrical current of the power supply 210 to track theamplification needs of amplifier module 134 during amplification of theaudio signals based on the power supply control signal.

The amplifier module 134 may output audio output signals on one or moreof the output audio channels 136, illustrated in FIG. 2 as a positiveand a negative side of the output channel(s), based on the processedaudio signal provided by the processing device 114. An output gain maybe applied to the processed audio signal by an output gain module 230 toadjust the levels of the output audio signals where the power amplifier126 has a predetermined fixed gain or amplification. Alternatively, theoutput gain module 230 may control the amount of gain or amplificationapplied to the output channels by the amplifier module 134, when theamplifier module 134 is capable of providing a variable gain oramplification of the output audio signals.

In FIG. 2, during example operation, the power supply control module 220may control the power supply 210 using the power supply control signalto maintain the supply power (VP) at a predetermined voltage, such as2.5VDC, using the feedback signal. During a monitoring period, whichcould be during operation or during a startup phase of the example audiosystem, such as when the vehicle is either started or placed in anaccessories mode, the processing device 114 can mute the processed audiosignals to reduce the output gain of the gain module 230 to zero therebyeliminating an audio output signal on the output audio channels 136 todrive the loudspeakers 106. During this time, as part of the STS and STGdiagnostics, the processing device 114 may monitor the feedback signalfor a possible STS fault in the form of a short to the power source ofthe audio system, or some other power source present in the vehicle.Alternatively, or in addition, the processing device 114 can perform themonitoring period during operation while the signal processor 104 isoutputting an audio output signal on the output audio channels 136 todrive the loudspeakers 106.

FIG. 3 illustrates an example STS fault scenario in which the positiveside of the output channel 136 is electrically shorted to a power sourceof the audio system, such as a 12 VDC power source, by a shortingconnection 302. During the monitoring period, such as the startup phaseand/or the operational phase of the audio system, the signal processor104 may perform fault diagnostics which include STS diagnostics. Duringthe fault diagnostics testing for STS, the signal processor 104 may mutethe processed digital audio signals on the processed digital audiosignal line 204. Muting may involve reducing the gain of the processeddigital audio signals to zero, attenuating the amplitude of theprocessed digital audio signals, or otherwise removing or minimizing allsignals from the processed digital audio signal line 204. Since theprocessed audio signals are muted by the processing device 114, thepotential of the positive side of the output audio channel 136 is raisedto the potential of the power source, in this example 12 VDC. The raisedpotential of the output audio channel 136 may pass through the amplifiermodule 134, as illustrated by arrow 304 and be sensed by the processingdevice 114 as the feedback signal on the feedback signal line 214exceeding a predetermined threshold such as 6.0VDC. Alternatively, thefault diagnostics may be performed during operation of the audio systemwithout muting the processed digital audio signals.

In response, the processing device 114 may perform fault detectionactivities. In one example, fault detection activities may includedisabling power supplied to the amplifier module 134, providing adiagnostics failure alarm and powering down the signal processor 104.The diagnostics failure alarm may include setting a diagnostic troublecode (DTC), sending an alarm indication, such as message displayed at ahead unit of vehicle to seek dealer service, or any other indication ofdetection of a short to power source (STS) situation. If the voltage ofthe power source of the audio system does not pass through the amplifiermodule 134, such as in a case where the amplifier module 134 includesreverse voltage protection, the processing device 114 may sense thevoltage at another location, such as on the output audio channels 136.

In the scenario of a short to ground (STG) fault, no electrical currentand voltage are supplied to the audio system 100 from a power supplyexternal to the audio system 100. Instead, excessive electrical currentis drawn from the audio amplifier 100 resulting in large electricalcurrent flow on the output audio channels 136. During operation of theaudio system 100 in the absence of a short to ground situation, however,relatively high electrical current may flow depending on the level ofamplification of the audio signals. Thus, to avoid erroneous detectionof a ground fault situation the audio fault detection system must becapable of discerning a ground fault situation.

FIG. 4 is a plot of a simulated effect of a zero ohm wire short toground potential on either the positive or the negative side of theoutput audio channel 136. In FIG. 4, a no AC signal output condition 402is illustrated as a reference. During the no AC signal output condition402, there is no audio output signal, short to power source, or short toground occurring on the output audio channels 136, and the output audiosignals remain at quiescent conditions (zero amps) for the illustratedtime duration of one second. A positive short impulse 404 illustrates apositive going alternating (AC) electrical current spike to about 0.5amps occurring about 0.55 seconds after the zero ohm short to groundpotential occurs on the positive side of the output audio channel 136.The positive impulse 404 then decays back to zero electrical current atabout 0.9 seconds after the zero ohm short to ground potential occurs. Anegative short impulse 406 illustrates a negative going AC electricalcurrent spike to about −0.5 amps occurring at about 0.52 seconds afterthe zero ohm short to ground potential of the negative side of theoutput audio channel 136. The negative impulse 406 then decays back tozero electrical current at about 0.9 seconds after the zero ohm short toground potential occurs. The electrical current may occur on the outputaudio channels 136 as power is applied to the amplifier module 134, suchas when the amplifier module 134 comes out of standby mode and applies apredetermined output voltage, such as 250 mV on each of the positive andnegative sides of the output audio channels 136. In other examples, thedecay rate of the positive and negative impulses 404 and 406 may bedifferent as a result of different inductive and capacitive propertiesof the audio system.

Accordingly, to detect a short to ground (STG) condition, the processingdevice 114 can identify an excessive electrical current draw during amonitoring period, such as during startup or operation of the audiosystem 100. In addition, operational conditions of the audio system 100can be accounted for by the processing device 114 to avoid incorrectdetection and indication of an STG condition by the processing device114.

One operational condition taken into consideration is the resistance ofthe loudspeakers 106. The loudspeakers 106 can represent a predeterminedresistive load. In some examples, the predetermined resistive load maybe as low as 2 ohms. The processing device 114 may detect resistive STGconditions, such as when there is some amount of resistance in the pathto ground, without falsely detecting the predetermined resistance of theloudspeakers as being a resistive STG condition. In this regard, theprocessing device 114 may detect resistive shorts to ground of up toabout one ohm.

In another operational condition, the processing device 114 maydifferentiate between a brief burst of voltage/current that may occur inthe audio system during a monitoring period, such as during systemstartup. Such a brief burst of voltage/current may occur, for example,when one or more of the loudspeakers vibrate due to external conditions,such as during a door slamming event in a vehicle. Vibration of aloudspeaker by an external force, such as due to an abrupt change in airpressure may result in reciprocation of a coil of the loudspeaker 106,which in turn may cause the loudspeaker 106 to resonate at a naturalresonant frequency and generate voltage/current. In other examples, anyother event, such as processing of audio content, such as music orvoice, through the audio system, may cause brief bursts ofvoltage/current on the output audio channels.

FIG. 5 is a plot of an example of a burst of electrical currentoccurring as a result of a vehicle door closing event for a loudspeaker106 in which voltage/current is generated. In the example of FIG. 5,generation of an alternating loudspeaker current 502 occurs within about0.45 seconds after a door is slammed in a vehicle in which theloudspeaker 106 is positioned. The loudspeaker 106 may, for example bepositioned in the door being closed, in another door of the vehicle, ina surface of the vehicle such as rear deck or dashboard, or any otherlocation. In this example, a peak amplitude of the electrical currentgenerated by the loudspeaker 106 during the door closing event is about+/−0.4 amps, which is similar to the peak electrical currents of theexample positive and negative short to ground potential eventsillustrated in FIG. 4. However, the loudspeaker electrical current 502oscillates around a natural frequency creating a number of positive andnegative current peaks as the electrical current decays in accordancewith the reduction in physical oscillations of the loudspeaker coiluntil the oscillations, and therefore generation of the loudspeakerelectrical current 502 become negligible at about 0.6 seconds after thedoor slam event. During the door slam event, the AC loudspeakerelectrical current 502 experiences a number of zero crossings due to theoscillations at resonance of the loudspeaker.

Thus, in comparing the zero crossings of the STG condition of FIG. 4 tothe door slam condition of FIG. 5, the longest duration of electricalcurrent 502 being generated on the output audio channels in which thereis no zero crossing during the door slam event is a period 504 of about60 milliseconds. In FIG. 4, on the other hand, due to the relativelyslow decay of the STG event, a period 408 without a zero crossing isabout 400 milliseconds. Since the electrical current waveform of FIG. 4is an exponentially decaying step response of AC electrical current,when an actual STG event occurs, the electrical current signal may notexperience a zero crossing until greater than a predetermined period oftime, such as about 100 milliseconds, whereas other conditions, such asa door slam event may experience a significant number of zero crossingsduring the same predetermined period.

FIG. 6 is a block diagram example of another audio processor 104 thatincludes a processing device 114 and a power amplifier 126. Theprocessing device 114 may provide a processed audio output signal to adigital-to-analog converter (DAC) 202 on a processed digital audiosignal line 206. The DAC 202 may produce an analog processed audiooutput signal on a power amplifier audio channel input line 206. The DAC202 may be included in the processing device 114, the power amplifier126, or may be a separate device. The power amplifier 126 may include apower amplifier module 134. The analog processed audio output signal onthe power amplifier audio channel input line 206 may be provided to thepower amplifier module 134 for amplification. The power amplifier module134 may increase the amplitude of the audio output signal by apredetermined fixed or variable amount, and output the amplified audiooutput signal(s) on the output amplifier channel(s) 136. The outputamplifier channel(s) 136 may be configured with a positive audio output(OUT+) and a negative audio output (OUT−).

The power amplifier 126 may also provide a feedback signal, in the formof an electrical current monitor output signal on a feedback signal line608. The feedback signal may provide an analog electrical currentfeedback signal to an analog-to-digital converter (ADC) 224. The outputof the ADC 224 may be provided as a digital signal to an electricalcurrent analysis module 614 included in the processing device 114 on adigital feedback signal line 612. The ADC 224 may be AC coupled to theamplifier module 134 such that only an AC component of the feedbacksignal is provided to the electrical current analysis module 614. In oneexample, the ADC 224 may include an onboard filter, such as a finiteimpulse response filter, to eliminate any DC components that may bepresent in the feedback signal, such as in an audio grade ADC. Inanother example, a capacitor, a filter or any other device may beprovided, such as in the electrical current feedback line 608 toeliminate any DC component that may be present in the electrical currentfeedback signal.

The electrical current feedback signal provided to the electricalcurrent analysis module 614 may provide an indication of the electricalcurrent present on the output audio channels 136. Accordingly, duringfault detection diagnostics, the signal processor 114 may use theelectrical current feedback signal and the electrical current analysismodule 614 during an STG event. The electrical current analysis module614 may capture electrical current feedback signal data in the form ofdigital samples of the electrical current feedback signal.

The electrical current analysis module 614 may perform STG diagnosticsby analysis of the digital samples of the alternating electrical currentof the current feedback signal during a monitoring period, such asduring operation or during startup of the audio system, such as when avehicle is started or the accessory feature is switched on. Part of thestartup or operation of the audio system involves the energizing orbringing the power amplifier 126 out of standby mode. In one example, asthe power amplifier 126 begins operation, a predetermined voltage, suchas 250 mV DC may be output on each of the output audio channels 136,resulting in a single-ended DC electrical current appearing on each ofthe output audio channels 136 (OUT+ and OUT−). The term “single ended”describes that the DC electrical current that appears on each of OUT+and OUT− of the output audio channels 136. In the event there is noshort to ground potential condition, the DC electrical current isfiltered such that digital samples received at the electrical currentanalysis module 614 are similar to the no AC signal output condition 402(FIG. 4). Alternatively, if there is a short to ground potential, an ACsignal appears on either the OUT+ or OUT of the output audio channels136 similar to the positive short impulse 404 or the negative shortimpulse 406. (FIG. 4) In still another alternative, depending onoperating conditions, an AC signal may appear on either the OUT+ or OUT−of the output audio channels 136 in the form of a burst of electricalcurrent, such as the alternating loudspeaker electrical current during adoor slam event in a vehicle (FIG. 5).

Upon detecting an AC signal on the output audio channels 136 during theSTG diagnostics, the electrical current analysis module 614 maydetermine if the AC signal is a short to ground potential event based onwhether the samples of AC signal received by the electrical currentanalysis module 614 are greater than a predetermined threshold during apredetermined window of time.

In one example, the predetermined threshold may be a value or magnitudeof electrical current samples that are greater than a predeterminedabsolute value (a positive or negative value). The value of thepredetermined threshold may be chosen to be outside a magnitude ofelectrical current drawn by a low resistance loudspeaker, such as a twoohm loudspeaker, but yet low enough of a value to recognize and identifya resistive short to ground, such as a one ohm short to ground, as wellas a low or substantially zero impedance short to ground condition inwhich a higher magnitude of electrical current can flow.

The predetermined window of time may be long enough to avoid detecting aburst of electrical current, such as an electrical current generated bya loudspeaker 106 during a door slam event or by audio content, such asmusic or speech driving a loudspeaker 106. Thus, in one example, basedon the example short to ground potential events of FIG. 4 and the doorslam event of FIG. 5, the predetermined period of time may be about 100milliseconds. In other examples, the predetermined window of time may belonger or shorter dependent on the duration of any expected burstcurrents and the duration of any expected short to ground electricalcurrents. In other words, the predetermined window of time may be longenough so that the electrical current analysis module 614 candistinguish between an electrical current developed on the output audiochannels 136 during a short to ground potential condition versus a burstof electrical current condition, and the electrical current exceeds thepredetermined threshold to distinguish between a ground fault electricalcurrent and electrical current present during startup or operation of alow impedance loudspeaker 106.

The electrical current analysis module 614 may include instructionsindependently executed by the digital signal processor 116 and themicroprocessor 118 (FIG. 1) to perform the STG diagnostics duringstartup and/or operation of the audio system. The digital signalprocessor 116 and the microprocessor 118 (FIG. 1) may cooperativelyoperate to efficiently perform the STG diagnostics, with each deviceperforming part of the STG diagnostics functionality. The digital signalprocessor 116 may perform the sampling of the electrical currentsamples. Thus, the predetermined period of time may be based on a numberof samples. For example, if the digital signal processor 116 operates ata clock speed of 48 kHz, the digital signal processor 116 may capturethe number of samples of the electrical current above the predeterminedthreshold for 4800 samples. The microprocessor 118 may perform the logicto determine if the predetermine threshold has been exceeded during thepredetermined period. For example, the microprocessor may confirm that avalue of each of the samples is above the predetermined thresholdthroughout the predetermined period of time. In other examples, theentirety of the STG diagnostics may be performed by only the digitalsignal processor 116 or the microprocessor 118. In addition, in otherexamples, counters, current sensors, registers, or any other techniquesmay be used by the processing device 118 to perform analysis of digitalsamples in view of the predetermined threshold electrical current andthe predetermined period of time.

In one example of the STG diagnostics performed by the electricalcurrent analysis module 614, a zero crossing analysis may also be usedduring the STG diagnostics in addition to using the predeterminedthreshold electrical current and the predetermined period of time. Azero crossing is the point in time when an alternating electricalcurrent present on the output audio channels 136 crosses a zero currentthreshold. Comparing zero current threshold crossings in FIG. 4 to FIG.5, the short to ground potential events (STG) resulting in the positiveshort impulse 404 or the negative short impulse 406 do not experience azero crossing for about 400 milliseconds. The burst currents generated,such as by audio content, or such as by a loudspeaker 106 as shown inFIG. 5, on the other hand, may experience a zero crossing in 60milliseconds or less.

In an example test case of a configuration of an audio system, it wasdetermined from test data that the period of time between zero crossingsduring various types of STG events ranging from a zero impedance shortto a one ohm short during different operational conditions was in arange of 143 milliseconds to about 200 milliseconds. In another exampletest case of the configuration of the audio system, it was determinedfrom test data that the period of time between zero crossings duringvarious burst of electrical current events was in a range of about 0 toabout 60 milliseconds. In addition, it was determined that thepredetermined threshold of electrical current for both a burst ofelectrical current event and a short to ground event with a shortcircuit of one ohm or less was above 0.4 amps. Thus, based on this testcase example, the predetermined threshold could be 0.4 amps, thepredetermined period of time could be greater than 60 milliseconds, andthe predetermined number of zero crossings distinguishing a burst ofelectrical current event from a short to ground event during thepredetermined period of time could be no zero crossings. In otherexamples, to account for variations in components, temperature and othervariables, the predetermined threshold could be 0.2 amps or in a rangeof 0.1 amps to 0.3 amps, the predetermined period of time could begreater than 100 milliseconds, or in a range of 80 to 120 milliseconds,and the predetermined threshold could be 5 zero crossings, or in a rangeof 3 to 8 zero crossings, for example. In other examples, otherpredetermined periods of time and predetermined threshold values may beused.

Referring again to FIG. 1, STG diagnostics, including zero crossingprocessing, may be performed using both the digital signal processor 116and the microprocessor 118. The computational strengths of each of thedigital signal processor 116 and the microprocessor 118 may be leveragedto perform the STG diagnostics to minimize communication bus traffic andoptimize operation of the digital signal processor 116 and themicroprocessor 118. In one example, the digital signal processor 116 mayperform the sampling, and operate a maximum zero crossing counter 142, alocal zero crossing counter 144, and the microprocessor 118 may performaccompanying logic 146 used in data analysis to determine if a short toground event occurs. The logic 146 may be stored in the memory 110.

In one example, the maximum and local zero crossing counters 142 and 144may each also include a respective maximum and local zero crossingcounter register that can be updated for each sample processed. In thisexample, the local zero crossing counter 144 may be incremented when azero crossing of the digital crossing electrical current samples isdetected by the digital signal processor 116, and the maximum zerocrossings counter 142 may count the total number of samples. Thus, themaximum zero crossing counter 144 can include a count of the number ofdigital samples processed during an electrical current event, and thelocal zero crossing counter 142 may indicate the number of zerocrossings during the electrical current event.

In another example, the maximum zero crossing counter 144 may store avalue of the maximum number of samples with a positive or negativemagnitude of electrical current that exceeds the predetermined thresholdvalue that occur between zero crossings, whereas the local zero crossingcounter 142 may store a count value of the number of samples since thelast zero crossing, that may be reset to zero whenever a zero crossingof the digital electrical current samples is detected by the digitalsignal processor 116. Thus, in this example, the maximum zero crossingcounter 144 can maintain a maximum number of samples processed betweenzero crossings, and the local zero crossing counter 142 may indicate thenumber of digital samples that have been processed since the last zerocrossing was detected. In other examples, the counters may be includedin the microprocessor 118, and the digital signal processor 118, or anyother processor included in the processing device 114.

Detection of the zero crossings may be performed based on analysis ofthe digital samples. In one example, the digital signal processor 116may perform a zero crossing calculation for each new sample:New Sample (NS)×Previous Sample (PS)=Result (R)  Equation 1where NS is a just received electrical current sample of the alternatingelectrical current present on one of the output audio channels 136, andthe PS is a previously received electrical current sample, such as theelectrical sample received just prior to the NS. Since the digitalsamples will be positive for positive electrical current and negativefor a negative electrical current samples, the digital signal processor116 can reset the local zero crossing counter 142 whenever R changessign. Thus, so long as R remains +R, the digital signal processor 116will not register a zero crossing, and/or reset the local zero crossingcounter 142, however, when +R becomes −R, the local zero crossingcounter 142 can be incremented and the register is updated, or is resetand the register is zeroed. The maximum zero crossing counter 144 may beincremented each time a digital sample is processed with a magnitude ofelectrical current outside the predetermined threshold, or when thevalue in the local zero crossing counter 142 exceeds the value presentlystored in the maximum zero crossing counter. In this way, the digitalsignal processor 116 may perform sample-by-sample zero crossing analysisand store an indication of the number of zero crossings and the numberof samples processed that exceed a predetermined threshold, and/or amaximum number of samples that have been processed between any number ofzero crossings.

In addition to using the predetermined threshold and the predeterminedperiod of time to identify a short to ground event, the microprocessor118 may also perform zero crossing based STG analysis. The additionalzero crossing analysis may be used to further determine if the event isa short to ground, or a burst electrical current. In one example, thismay involve determining if the zero crossing count of the zero crossingcounter exceeds a predetermined zero crossing count value within thepredetermined period of time while the predetermined threshold is beingexceeded. In another example, this may involve determining if the numberof samples, or a predetermined time, between zero crossings exceeds apredetermined maximum number while the predetermined threshold is beingexceeded.

In one example, the digital signal processor 116 may buffer a maximumnumber of zero crossings in the local zero crossings counter 142, andthe total number of samples in the maximum zero crossings counter 144,and the microprocessor 118 may check if a predetermined maximum numberof zero crossings (predetermined value) has been exceeded after apredetermined period of time indicated by the number of samples (delayfollowing commencement of the electrical current event). In anotherexample, the microprocessor 118 may monitor for a reset of the zerocrossing counter 144, or a count of a number of zero crossings, whilethe feedback electrical current is above the predetermined threshold,and determine if a period of time since the last time (or a determinednumber of times) the zero crossing counter was reset exceeds thepredetermined period.

In another example, the digital signal processor 116 may store thenumber of samples between zero crossings that are outside thepredetermined threshold in the local zero crossing counter 142, andbuffer the maximum number of samples outside the threshold between zerocrossings in the maximum zero crossings counter 144. In this example,the microprocessor 118 may check if a predetermined maximum number ofsamples between zero crossings (predetermined value) has been exceeded.

Thus, in the example of occurrence of a burst of electrical currentusing the previously discussed testing example, zero crossing countscould be added at a maximum of every 60 milliseconds, or 2880 sampleswith a 48 kHz sample frequency, whereas in the example of occurrence ofa short to ground event, zero crossing counts could be added at aminimum of every 143 milliseconds, or 6864 samples. In other words, inthe example of occurrence of a burst of electrical current using thepreviously discussed testing example, the samples between zero crossingscould be a maximum of every 60 milliseconds, or 2880 samples with a 48kHz sample frequency, whereas in the example of occurrence of a short toground event, the samples between zero crossings could be at a minimumof every 143 milliseconds, or 6864 samples.

In still other examples, the microprocessor 118 could receive a count ofthe number of samples from the digital signal processor 116, and monitorzero crossing counter 142 to be reset, or check for a change in thenumber of zero crossings present in the zero crossings counter 142 at apredetermined number of samples, such as when 2880 or 6864 samples werereached. In still other examples, any other logic could be performed bythe microprocessor 118 in any other way to establish an STG event usingthe local and/or maximum zero crossing counters 142 and/or 144, thepredetermined threshold, and/or the predetermined period of time. Inaddition, in still other examples, any other form of cooperativeoperation between the digital signal processor 116 and themicroprocessor 118 may be implemented to determine an STG event using atleast one of the local zero crossing counter 142, the maximum zerocrossing counter 144, the predetermined threshold, and the predeterminedperiod of time. If the local and maximum zero crossing counters 142 and144 or the sample counter are running all the time the audio system isoperating, the local and maximum zero crossing counters 142 and 144and/or the sample counter may be reset, or a value may be logged at thecommencement of startup of the power amplifier 126 and/or when STGdiagnostics are initiated.

FIG. 7 is an example operational block diagram that includes themicroprocessor 118, the digital signal processor (DSP) 118, and thepower amplifier 126, and illustrates an example distribution of thefunctionality of the STG diagnostics between the microprocessor 118 andthe DSP 116. In FIG. 7, operation begins at block 702 when the audiosystem is energized, such as when an ignition switch of a vehicle ischanged to an “on” position to start the vehicle or enter an accessorymode. In other examples, any other condition may trigger the operation,such as a predetermined time or event that occurs, while the audiosystem is operating to process audio content, such as music or voice.The ignition on event may be sensed by the microprocessor 118 at block702. At block 704, the microprocessor 118 generates and transmits aninitialization message to the DSP 116. Communication between themicroprocessor 118 and the DSP 116 may be over a control line, adedicated bus, a bus network, such as a CAN bus or a MOST bus, or viaany other communication path, and may include hardware and/or software.Alternatively, the microprocessor 118 and the DSP 116 may be a singledevice, or multiple devices of the same device type cooperativelyoperating. In the case of a single device, the initialization messagecould be a software event, such as a flag, a register, a variable, orany other notification mechanism.

Upon receipt of the initialization message, the DSP 116 initiatesoperation and loads program instructions, including the STG diagnosticsfunctionality at block 708. The DSP 116 initiates diagnostics, such asby clearing stored DSP variables from memory at block 710. Clearing ofstored variables may include resetting registers and counters, such as asample counter register, and the local and maximum zero crossing counterregisters. Alternatively, clearing stored variables may be identifyingvalues present in the registers and counters as starting values, ratherthan zeroing the registers and counters. At startup, the microprocessor118 communicates with the DSP 116 to confirm the DSP is running at block714. Upon completion of startup of the DSP 116, the DSP 116 maycommunicate to the microprocessor 118 that startup was successfullycompleted, or the microprocessor 118 may query the DSP 116 to confirmstartup was completed successfully. If the DSP 116 is not running, theoperation returns to block 704 and the microprocessor 118 re-transmitsthe initialization message to the DSP 116. In other examples, the DSP116 may initialize the microprocessor 118.

If, at block 714, the DSP 116 is running, the microprocessor 118transmits an energization message to the power amplifier 126 at block716. Communication between the microprocessor 118 and the poweramplifier 126 may be over a control line, a dedicated bus, a busnetwork, such as a CAN bus or a MOST bus, or via any other communicationpath that may include hardware and/or software. Upon receiving theenergization message, the power amplifier 126 may startup, emerge from astandby state, or otherwise supply a predetermined voltage on the outputaudio channels. In other examples, the power amplifier 126 may bestarted up by the DSP 116, or any other device, or may already beenergized and in operation. At block 718, the microprocessor 118 maycommunicate instructions to the DSP 116 to mute the audio signals beingprovided as output audio signals to the power amplifier 126. The DSP 116may mute, turn off, or otherwise disable output of processed audiosignals by the processing device 114 to the power amplifier 126 at block720 such that the power amplifier 126 is no longer performingamplification of audio signals. In other examples, such as duringoperation of the audio system, the muting of the processed audio signalsmay be omitted as part of the operation of the STG diagnostics, and theSTG diagnostics may be performed while audio content is being output bythe audio system.

At block 722, the microprocessor 118 may delay taking any activity, andawait further processing by the DSP 116. The microprocessor 118 may, forexample, wait until the power amplifier 126, such as a power IC, isfully biased (account for worst-case stack ups). The DSP 116 may processsamples of the electrical current feedback signal to monitor the outputaudio channels for occurrence of an electrical current event for eachchannel at block 724.

FIG. 8 is an example flow diagram of the processing of a digital sampleby the DSP 116 during monitoring of the output audio channels for anelectrical current event using the electrical current feedback signal asdescribed in block 724. (FIG. 7) In FIG. 8, the DSP 116 may monitor theelectrical current on the feedback current line 612 (FIG. 6), anddetermine if the absolute value of the electrical current (I) of thesample for an audio channel is greater than a predetermined startthreshold electrical current, which in one example can be 0.4 amps. Thepredetermined start threshold electrical current may be anypredetermined value that is greater than 0.2 amps to indicate that thereis a new electrical current event present on the output audio channels.Accordingly, the electrical current on the output audio channelsexceeding the predetermined threshold provides a start time of thepredetermined period of time within which STG diagnostics are performedfor a series of electrical current samples. If the electrical current(I) is not greater than the starting threshold electrical current, it isdetermined if an event register is equal to one at block 803. If atblock 803, the event register is not set equal to one, the eventregister remains set equal to zero. If, on the other hand, theelectrical current (I) of the present sample is greater than thepredetermined threshold electrical current, at block 806 the eventregister is set equal to one indicating an event is occurring that couldbe subject to the STG diagnostics. Thus, as alternating electricalcurrent samples are being processed by the DSP 116, and an event occurs,the event register may indicate the event and not be reset untildiagnostics are again initiated (block 710 of FIG. 7). Since sampling bythe DSP 116 occurs at a predetermined frequency, such as 48 kHz, themaximum number of samples may be used to determine the maximum timebetween zero crossings

In addition, at block 808, the DSP 116 may perform zero crossinganalysis using Equation 1 to multiply the current sample times theprevious sample to obtain an output (y). It is determined if the output(y) is less than or equal to zero at block 810. As previously discussed,the output (y) will be positive when the previous sample and the currentsample are both negative, or both positive, thereby indicating that nozero crossing has occurred. However, upon output (y) being a negativevalue, it is indicated that there was a transition between a positiveelectrical current and a negative electrical current between the currentsample and the previous sample. In other words, a zero crossing hasoccurred between the previous sample and the current sample. If a zerocrossing has not occurred based on output (y) being positive, the localzero crossing counter is incremented at block 812 by the value of theevent register, which will be set to either a “1” or a “0” depending onwhether an event is occurring. By incrementing the local zero crossingcounter using the value of the event register, the local zero crossingcounter will not increment unless an event is occurring (event registerset=“1”). If, on the other hand, a zero crossing has occurred, andoutput (y) is negative, the local zero crossing counter is zeroed atblock 814.

At block 816, it is determined if the local count value stored in thelocal zero crossing counter is greater than a maximum zero crossingcount value stored in the maximum zero crossing counter. If the localcount value is greater than the maximum zero crossing count value, themaximum zero crossing count value is set equal to the local count valueat block 818, and the processing of the sample is complete at block 820.If, on the other hand, the maximum zero crossing count value is greaterthan the local count value at block 816, the processing of the sample iscomplete at block 820 without changing the maximum zero crossing countvalue. The local count value is incremented for each digital sample,whereas the maximum zero crossing counter represents only the largestnumber of samples that have been processed without occurrence of a zerocrossing for a respective audio channel. Accordingly, in this example,the DSP 116 counts and stores 1) the current number of samples in thelocal zero crossing counter 142 since the last zero crossing occurrence;and 2) a maximum number of samples between zero crossings, which isstored in the maximum zero crossing counter 144. Alternatively, aspreviously discussed, the DSP 116 may store 1) the total number ofsamples in maximum zero crossing counter 144; 2) whether a sampleindicates a zero crossing; and/or 3) the total number of zero crossings.

The sample processing by the DSP 116 may occur continuously duringoperation of the audio system. Thus, STG diagnostics may occur atstartup, or anytime during operation of the audio system. STGdiagnostics may occur whenever the initiate diagnostics operationoccurs. (Block 710, FIG. 7) In addition, STG diagnostics may beperformed on one or multiple output audio channels in series or inparallel. In the case of multiple output audio channels, the DSP 116 mayinclude local registers and maximum zero crossing registers thataccumulate values separately for each of the corresponding output audiochannels. Thus, each of the output audio channels that undergo STGdiagnostics can have a local register and a maximum zero crossingregister with accumulated values during the STG diagnostic event. Forexample, in the case of an integrated circuit power amplifier thatincludes two or four channels providing amplified audio signals onoutput audio channels, the STG diagnostics can be performed in parallelfor the two or four channels.

Referring again to FIG. 7, at block 726, the microprocessor 118 may readthe counters and other data from the DSP 116, including the local zerocrossing register and/or the maximum zero crossing register for each ofthe one or more output audio channels. The registers may be read by themicroprocessor 118 after a predetermined number of samples, at theconclusion of the predetermined period of time, periodically during thestartup, or upon any other trigger or timed event during the startup oroperation of the power amplifier 126. In this example, the maximum zerocrossing counter register may include the maximum number of samplesbetween zero crossings that occurred during a period of time when theabsolute value of the current is above the predetermined thresholdcurrent. Alternatively, or in addition, the maximum zero crossingcounter register may include the maximum count of zero crossings thatoccurred during the predetermined period of time represented by thesample counter register. The microprocessor 118 may compare the count ofthe maximum number of samples between zero crossings (during thepredetermined period of time) to a predetermined threshold at block 726.The predetermined threshold may be stored in memory 110, and thepredetermined period of time may be represented by, or determined basedon, the number of samples in the sample count register.

FIG. 9 is an example flow diagram of the logic operations of themicroprocessor 118 during comparison of the maximum zero crossings countto the predetermined threshold. In FIG. 9, the microprocessor 118determines if the maximum zero crossing count value is greater than apredetermined maximum zero crossing count value for an audio channel atblock 902. In one example, a number of samples in the maximum zerocrossings counter representing a maximum period of time between zerocrossings is compared to a predetermined number of samples representingthe predetermined period of time. In another example, the number of zerocrossings during a predetermined period of time represented by acorresponding number of samples in the maximum zero crossing counter (aperiod of time) is compared to the predetermined number of samplesrepresenting the predetermined period of time. In the case of themaximum zero crossing value representing the maximum time between zerocrossings, if the maximum zero crossing count value is less than thepredetermined threshold, the microprocessor 118 may determine there isno ground fault at block 904. In the case of the maximum zero crossingcount value being the total number of samples and the local zero countvalue being the number of zero crossings, if the local zero crossingcount value exceeds the predetermined zero crossing count value withinthe samples representing the predetermined period of time provided bythe maximum zero crossing count value, the microprocessor 118 maydetermine there is no ground fault detected on the corresponding outputaudio channel at block 904. At block 906, the microprocessor 118determines if STG diagnostics have been performed for all output audiochannels of the power amplifier 126. If STG diagnostics have not yetbeen performed for all the output audio channels, the microprocessor 118selects another of the output audio channels at block 908 and returns toblock 902 to determine if the maximum zero crossing count valuerepresenting samples between zero crossings, or representing the numberof samples used with corresponding zero crossing count value, of theselect output audio channel exceeds the predetermined threshold. If, onthe other hand, at block 906 it is determined that STG diagnostics havebeen performed on all output audio channels, the microprocessor 118determines that there are no ground faults present on any output audiochannel, and unmutes the processed audio signals at block 910, if duringstartup. During operation, the operation of block 910 may be omitted. Atblock 912, the audio system begins (or continues) operation byprocessing, amplifying and outputting audio content on the output audiochannels to drive loudspeakers.

If at block 902, it is determined by the microprocessor 118 for anoutput audio channel that the electrical current is above thepredetermined threshold for the predetermined period of time, and numberof samples between zero crossings is greater than the predeterminedthreshold, or the zero crossing count is less than the predeterminedzero crossing count value within the samples representing thepredetermined period of time, the microprocessor 118 determines that aground fault is present on that output audio channel at block 914. Atblock 916, the microprocessor 118 transmits a de-energize message to thepower amplifier 126, and the power amplifier 126 subsequent powers down.The microprocessor 118 then outputs a fault condition alarm, such as adiagnostics failure alarm at block 918. The fault condition alarm may,for example, be transmitted over a communication bus to a user display,and to other devices in the audio system.

Referring again to FIG. 7, at block 730, the microprocessor 118 mayinitiate a reset in preparation for another diagnostic. The reset mayinclude communicating a message to the digital signal processor 116 toinitiate diagnostics (block 710). At block 732, the microprocessor 118may wait for a predetermined period of time, to allow the digital signalprocessor 116 to capture and store additional counts in the counters andregisters, before the microprocessor 118 returns to block 726 to readthe DSP counters and registers. In addition, the predetermined wait timemay be set to allow the microprocessor 118 to perform other functions,such as other functions in the audio system, or the signal processor.

In the previously discussed examples, an audio system having a signalprocessor capable of performing STG diagnostics on any number of audiochannels may operate solely in the digital domain. The signal processormay include a DSP executing instructions to perform sampling of digitalsamples representative of an alternating electrical current signalpresent on output audio channel(s), and a microprocessor executinginstructions to analyze the sampling performed with the DSP. The STGdiagnostics may be performed by the cooperative operation of the DSP andmicroprocessor on any number of output audio channels during a startupphase, or during operation of the signal processor. During the STGdiagnostics, a ground fault may be detected within a predeterminedperiod of time using a predetermined threshold. In one example, the STGdiagnostics may include an analysis of the time between zero crossingsto determine if a ground fault is present based on the predeterminedperiod of time. In another example, the STG diagnostics may includeanalysis of a number of zero crossings exceeding a predetermined zerocrossing count value within the predetermined period of time. Thepredetermined threshold may be a predetermined absolute value of the ACaudio signal, and the predetermined period of time may be based on apredetermined number of samples processed by the DSP. Thus, the signalprocessor may identify a ground fault on any of a number of differentaudio channels during startup when an electrical current present on anyone of the audio channels exceeds the to predetermined threshold duringthe predetermined period of time. In addition, the signal processor mayidentify a ground fault on any of a number of different audio channelsduring operation and during startup when a number of samples betweenzero crossings on any one of the audio channels exceeds a predeterminedthreshold value when the predetermined threshold is being exceeded.Alternatively, or in addition, the signal processor may identify aground fault on any of a number of different audio channels duringoperation and during startup when a number of zero crossings on any oneof the audio channels is less than the predetermined zero crossing valueduring the predetermined period of time when the predetermined thresholdis being exceeded.

In addition to the previously discussed examples, other exampleconfiguration and operations are possible. For example, the signalprocessor may include one or more processors that are the same or aredifferent, and the allocation of the functionality described herein maybe distributed among the one or more processors. Also, use of the zerocrossings to detect ground faults on the audio output channels usingexecutable code may be performed in different ways, and/or usingdifferent numbers and kinds of registers. Thus, while variousembodiments of the invention have been described, it will be apparent tothose of ordinary skill in the art that many more embodiments andimplementations are possible within the scope of the invention.Accordingly, the invention is not to be restricted except in light ofthe attached claims and their equivalents.

I claim:
 1. An audio channel fault detection system comprising: aprocessing device configured to digitally process audio signals; and apower amplifier coupled with the processing device, the power amplifierconfigured to amplify audio signals digitally processed by theprocessing device and output amplified audio signals on an output audiochannel to drive a loudspeaker; the processing device configured toexecute instructions to detect a short to ground event based on anamount of samples of alternating electrical current on the output audiochannel that have zero crossings being less than a predetermined amountof zero crossings for a predetermined window of time, and the processingdevice configured to count a number of samples between a first zerocrossing and a second zero crossing of the alternating electricalcurrent, the short to ground event detected with the processing devicein response to the number of samples being greater than a predeterminednumber of samples representative of the predetermined window of time. 2.The audio channel fault detection system of claim 1, where thepredetermined window of time is about 100 milliseconds or greater. 3.The audio channel fault detection system of claim 1, where theprocessing device is configured to execute the instructions to detectthe short to ground during startup of the power amplifier, theprocessing device further configured to mute output of digitallyprocessed audio signals to the power amplifier during detection of theshort to ground.
 4. The audio channel fault detection system of claim 1,where the processing device is configured to execute the instructions todetect the short to ground during startup of the power amplifier orduring operation of the power amplifier.
 5. The audio channel faultdetection system of claim 1, where the processing device includes adigital signal processor, the digital signal processor configured tostore a count of a number of zero crossings of the samples ofalternating electrical current during the predetermined window of time.6. The audio channel fault detection system of claim 5, where theprocessing device further comprises a microprocessor, the microprocessorconfigured to determine if the count of zero crossings is less than apredetermined maximum number of zero crossings during the predeterminedwindow of time to detect the short to ground event.
 7. The audio channelfault detection system of claim 5, where the digital signal processoriteratively multiplies a current sample times a previous sample toobtain a result, and the digital signal processor is configured toincrement a zero crossings counter each time the result changes from apositive value to a negative value.
 8. The audio channel fault detectionsystem of claim 1, where the processing device includes a digital signalprocessor, the digital signal processor configured to monitor a count ofthe number of samples of alternating electrical current that occursequentially without a zero crossing, and to update a value of a maximumzero crossings counter in response to the value being exceeded by thecount.
 9. The audio channel fault detection system of claim 1, where theprocessing device further comprises a microprocessor, the microprocessorconfigured to determine if the maximum zero crossings counter contains anumber of samples that is greater than a predetermined maximum number ofsamples to detect the short to ground event.
 10. A non-transitorycomputer readable medium that stores instructions executable by aprocessor to perform audio channel fault detection, the computerreadable medium comprising: instructions executable with the processorto digitally process an audio input signal and generate a digitallyprocessed audio output signal; instructions executable with theprocessor to supply the digitally processed audio output signal to apower amplifier used to amplify the digitally processed audio outputsignal; instructions executable with the processor to sample analternating electrical current of the amplified digitally processedaudio output signal on an output audio channel of the power amplifier;instructions executable with the processor to determine if samples ofalternating electrical current on the output audio channel have zerocrossings for a predetermined period of time; instructions executablewith the processor to indicate a ground fault event has occurred on theoutput audio channel in response to an amount of the samples ofalternating electrical current on the output audio channel that havezero crossings being less than a predetermined amount of zero crossingsfor the predetermined period of time; instructions executable with theprocessor to determine a number of samples between a first zero crossingand a second zero crossing of the alternating electrical current; andinstructions executable with the processor to indicate a ground faultevent has occurred on the output audio channel in response to the numberof samples between the first and second zero crossings being greaterthan a predetermined number of samples.
 11. The computer readable mediumof claim 10, where the instructions executable with the processorfurther comprise instructions to determine a maximum number of digitalsamples that are between zero crossings of the alternating electricalcurrent on the output audio channel, and determine if the maximum numberof digital samples exceeds the predetermined period of time.
 12. Thecomputer readable medium of claim 11, where the instructions executablewith the processor further comprises instructions to reset a count ofthe number of the samples between each zero crossing in response to azero crossing, and instructions executable with the processor update avalue of a maximum zero crossing counter with the count in response tothe count being greater than the value.
 13. The computer readable mediumof claim 10, where the instructions executable with the processorfurther comprise instructions to store a count of the number of zerocrossings of the alternating electrical current on the output audiochannel during the predetermined period of time that the predeterminedthreshold is being exceeded.
 14. The computer readable medium of claim13, where the instructions executable with the processor furthercomprise instructions to indicate the ground fault event has occurred inresponse to the stored count of the number of zero crossings being lessthan a predetermined number of zero crossings at a conclusion of thepredetermined period of time.
 15. The computer readable medium of claim10, where the instructions executable with the processor furthercomprises instructions executable with the processor to increment a zerocrossing register each time a zero crossing is detected among thesamples of alternating electrical current, and instructions executablewith the processor to indicate the ground fault event in response to acount of the register being less than a predetermined number after thepredetermined period of time.
 16. The computer readable medium of claim10, where the instructions to sample the alternating electrical currenton the output audio channel of the power amplifier are configured forexecution with a digital signal processor, and the instructionsexecutable to determine if the samples of alternating electrical currenton the output audio channel exceed the predetermined value for thepredetermined period of time are configured for execution with amicroprocessor, the computer readable medium further comprisinginstructions executable with the digital signal processor and themicroprocessor to communicate therebetween over a communication bus. 17.A method of performing audio channel fault detection, the methodcomprising: energizing a power amplifier; digitally sampling a feedbackelectrical current of the power amplifier, the feedback electricalcurrent indicative of an electrical current present on an output audiochannel of the power amplifier; determining a number of samples betweenzero crossings of the feedback electrical current; and identifying aground fault condition in response to the number of samples between zerocrossings being greater than a predetermined value.
 18. The method ofclaim 17, where determining the number of samples between zero crossingscomprises initiating a count of the number of samples in response to thefeedback electrical current going outside a predetermined threshold. 19.The method of claim 17, where determining a number of samples betweenzero crossings comprises multiplying an electrical current sample by aprevious sample, and resetting a count of the number of samples betweenzero crossings each time a result of the multiplication is a negativevalue.
 20. The method of claim 19, where resetting a count of the numberof samples between zero crossings further comprises updating a maximumzero crossings counter to include the count when a value of the maximumzero crossing counter is less than the count, and comparing the value tothe predetermined value to identify the ground fault condition.
 21. Themethod of claim 17, where digitally sampling a feedback electricalcurrent of the power amplifier comprises independently sampling thefeedback electrical current of the power amplifier for each of aplurality of output audio channels, the feedback electrical current foreach of the plurality of audio output channels indicative of therespective electrical current present on each of the output audiochannels of the power amplifier.